in small utility engines, each throttle bore (also known as a barrel) of a multiple throttle bore carburetor generally provides air and fuel to only one cylinder. For example, a two-barrel carburetor would generally be used to fuel a two cylinder engine. Throttle bores are generally formed into a common component known as a carburetor body. Gaseous fuel carburetors (as opposed to the more common liquid fuel carburetors, such as gasoline) are used when an engine is designed to run on a fuel that is delivered to the ermine in gaseous form, such as natural gas or propane.
There are types of gaseous fuel carburetor designs that introduce fuel to the carburetor air stream via a venturi passage that is created when a “hollow” venturi tube is pressed or otherwise fitted into a throttle bore just upstream of the throttle plate/throttle shaft assembly. The venturi tube in this type of carburetor design is the component that meters fuel into the carburetor air stream by restricting the air flow slightly via a throat section, thereby creating a pressure differential that draws fuel from a fuel delivery passage of the carburetor body through a fuel port (which can consist of a plurality of metering orifices) formed in the wall of venturi tube.
Certain variations in engine design and construction (such as the 90 degree V-Twin configuration) have differing fuel requirements for each cylinder in order to run at optimum conditions in each cylinder. For multiple throttle bore carburetors, the prior art is such that each venturi tube is independent from the others, not only in function, but in construction as well. The venturi tubes can be made of a variety of materials and can have one or more metering orifices formed into the walls of the venturi tubes that can vary in size, circumferential and/or axial location relative to the venturi perimeter, and even shape.
Therefore, it becomes important for consistency of fuel delivery to the combustion chambers of the cylinders that these independent venturi tubes be fitted into the appropriate throttle bore, in the correct spatial orientation, and that they stay in the correct spatial orientation for the life, of the carburetor.